This invention relates to transmission line-interface circuits, also known as line circuits or port circuits.
A telephone line is terminated by a line-interface circuit at each end. A line-interface circuit both transmits and receives on the line. The transmitter of a conventional digital line-interface circuit has a high impedance when turned off and a low impedance when turned on. A network of series and shunt resistors is typically used to match the output impedance of the transmitter to the input impedance of the line. The network is typically designed to provide a matched impedance when the transmitter is turned off. This results in the line being driven with a source impedance that is less than the line impedance when the transmitter is turned on. This impedance mismatch results in undesirable signal reflections, as is well known. Moreover, the shunt resistors result in a waste of power. In most cases, about 50% of the transmitter""s output power is absorbed by the shunt resistors.
This invention is directed to solving these and other problems and disadvantages of the prior art. Illustratively according to the invention, a line-interface circuit uses op amps to create a virtual short or ground at the output of the transmitter when the transmitter is turned off. This maintains a substantially constant impedance at the output of the transmitter irrespective of whether the transmitter of turned on or off. A network of series resistors is then used to match the impedance of the line. Since the output impedance of the transmitter is constant at all times, the impedance of the linexe2x80x94interface circuit is matched to the line impedance at all times by the resistor network. Moreover, since the resistor network does not use shunt resistors, the waste of power that is caused by the shunt resistors in conventional line interface circuits is eliminated. Furthermore, an illustrative implementation of the line-interface circuit uses fewer components than the implementation of a conventional line-interface circuit.
Generally according to the invention, a line-interface circuit comprises a transmitter and a receiver as follows. The transmitter presents a high impedance to a telephone line when the transmitter is not transmitting and a low impedance when it is transmitting, as is conventional. Significantly, however, the receiver is coupled to the transmitter and presents a high impedance to the telephone line when the transmitter is transmitting and a low impedance when the transmitter is not transmitting, so that an operating line-interface circuit presents a substantially constant impedance to a connected telephone line at all times. Illustratively, the receiver includes a switching arrangement that electrically couples the receiver to the telephone line when the transmitter is not transmitting and electrically uncouples the receiver from the telephone line when the transmitter is transmitting, and circuitry that connects to the telephone line through the switching arrangement and emulates shorting or grounding of the telephone line. Preferably, the switching arrangement comprises either a pair of transistors each coupling the receiver to a different lead of the telephone line, or enable inputs on the emulating circuitry and a signal line connected to the enable inputs that respectively enables and disables the emulating circuitry as the transmitter is disabled and enabled. Further preferably, the line-interface circuit comprises circuitry for coupling the transmitter and the receiver to the telephone line without employing shunt resistance across the telephone line.
These and other features and advantages of the present invention will become apparent from the following description of illustrative embodiments of the invention considered together with the drawing.